[cfe-dev] ASTMatchers.h and its internal linkage variable definitions

[David Blaikie via cfe-dev]

On Wed, Nov 15, 2017 at 12:20 AM Manuel Klimek <klimek at google.com> wrote:

> On Wed, Nov 15, 2017 at 9:18 AM Manuel Klimek <klimek at google.com> wrote:
>
>> On Tue, Nov 14, 2017 at 9:26 PM David Blaikie <dblaikie at gmail.com> wrote:
>>
>>> On Tue, Nov 14, 2017 at 1:04 AM Manuel Klimek <klimek at google.com> wrote:
>>>
>>>> On Tue, Nov 14, 2017, 12:53 AM David Blaikie <dblaikie at gmail.com>
>>>> wrote:
>>>>
>>>>> Ping?
>>>>>
>>>>> Richard & I decided that supporting this particular case of
>>>>> namespace-scope static variables in modular headers using modular codegen.
>>>>>
>>>>
>>>> Part of the sentence is missing?
>>>>
>>>
>>> Huh, right you are...
>>>
>>> What I was going to say (but lost track through all the
>>> caveats/criteria): Richard & I decided that <stuff> is invalid (it's a
>>> legit ODR violation) & doesn't need to be supported.
>>>
>>> (though there may still be issues with the non-ODR violation - such as
>>> the use of internal linkage namespace-scope reference variables... :/ )
>>>
>>>
>>>>
>>>> (specifically the case where the namespace-scope static variable is
>>>>> referenced from an inline function - thus making a true ODR violation (in
>>>>> classic C++ this'd be an ODR violation if more than one TU used that header
>>>>> - so it seems OK to not support this in modular codegen))
>>>>>
>>>>
>>>> Calling is not an odr use, right? And I thought referencing alone
>>>> doesn't lead to an odr violation?
>>>>
>>>
>>> This is what the ASTMatcher code boils down to:
>>>
>>> struct foo {
>>>   foo();
>>>
>>
> Ah, here's the detail I missed: it actually boils down to
> struct foo {
>   ExpressionTemplateMatcherType operator()(...);
> }
> The only use of foo is by calling the operator on it.
>

Right, yep yep (sorry I skipped over a few too many steps in my
reduction/example). Calling a non-static member function is an ODR use,
roughly equivalent to passing the value by reference to another function
(much the same as returning by reference).

- Dave


>
>
>> };
>>> const foo f; // this has internal linkage
>>> const foo &x() { return f; }
>>>
>>
>> Where do we return matcher reference types?
>> This should just return foo by value?
>>
>> if 'x' appears in more than one TU, it's an ODR violation because 'f'
>>> resolves to different entities in each TU so there are multiple distinct
>>> definitions of 'x'.
>>>
>>> In this case, for example, 'f' is 'callExpr' (ASTMatchers.h:1138) and
>>> 'x' is the AST_POLYMORPHIC_MATCHER_P2 on ASTMatchers.h:3491 that uses
>>> 'callExpr' in an inline function in the header.
>>>
>>
>> That just calls callExpr() though, and never ODR uses it?
>>
>>
>>> If the namespace-scoped static variable is unreferenced from inline
>>>>> functions, that's fine - such as the iostreams global initializer.
>>>>>
>>>>> I'll send a CR to move these definitions out of line.
>>>>>
>>>>
>>>> I'm happy with that, too. I assume once c++17 hits the shelves we'll be
>>>> able to replace it anyway?
>>>>
>>>
>>> With inline variables? Yeah, I think so. Though I should test/figure out
>>> whether/how modular codegen will work with inline variables...
>>>
>>>
>>>>
>>>> On Thu, Nov 9, 2017 at 12:30 AM Manuel Klimek <klimek at google.com>
>>>>> wrote:
>>>>>
>>>>>> On Thu, Nov 9, 2017 at 12:43 AM David Blaikie <dblaikie at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Hey Manuel,
>>>>>>>
>>>>>>> In an effort to apply Modular Code Generation to Clang internally, I
>>>>>>> came across the fact that this header contains a bunch of internal linkage
>>>>>>> variables. (now, maybe modular code generation should be compatible with
>>>>>>> internal linkage namespace-scoped variables... I'm looking into that)
>>>>>>>
>>>>>>> These variables technically create ODR violations any time they're
>>>>>>> ODR-used in another inline function used in more than one translation unit.
>>>>>>>
>>>>>>> Is there a good way we could fix this header so it doesn't create
>>>>>>> ODR violations (hopefully by not containing internal linkage entities)? I
>>>>>>> mean one simple solution is to declare all these things in a header and
>>>>>>> define them out of line. Since they have no actual state, that's probably
>>>>>>> pretty low-cost except for a lot of duplication. I'm happy to use a
>>>>>>> .def/.inc file to help remove a bunch of that duplication, if useful.
>>>>>>>
>>>>>>
>>>>>> I remember having that argument a long time ago with somebody
>>>>>> (perhaps Sam (cc'ed)?)
>>>>>>
>>>>>>>
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